The present invention relates to a sintered, alumina ceramic product. More particularly, the present invention relates to a sintered, alumina ceramic product exhibiting a high homogenity of performance between products produced in the same batch.
As is known, the properties of ceramic pieces are in most cases non-uniform, i.e., the value of a property, for example hardness, varies from point to point of a specimen and even more so between different products produced in the same batch.
These variations are due to variations in the density, phase composition, grain size, porosity and internal stress distribution of the products. There are many factors all along the manufacturing process which are responsible for the variation of the characteristics mentioned above. For instance, a component made by pressing and having a complex shape has a non-uniform bulk density after pressing. This is so because the applied pressure is not identical at all points when the shape of the desired product is complex. In many cases these xe2x80x9cgreenxe2x80x9d density differences remain also after the sintering of the product.
Another problem contributing to the non-uniformity is the temperature gradients in the furnaces used for sintering. These will cause differences from product to product, or within a given product in the density thereof and, as a consequence, in the properties thereof. Other problems are related to non-uniform mixing of the initial raw materials, e.g., Al2O3 and the sintering aids.
This also leads to fluctuations in the fired part density. The sintering process itself, even without temperature gradients, is non-uniform also due to the aggregates present in the powder to be sintered. This may cause differences from point to point in the product itself, in the size and numbers of the residual pores which remain after sintering. In other cases rapid cooling may leave stresses which are not evenly distributed. Such stresses also influence properties such as strength.
Thus, e.g., at a conference being organized for Apr. 30-May 3rd, 2000 at America""s Center, St. Louis, Mo., Focused Session 6 is entited: Point Defects, Transport, and other Defect-Related Phenomena in Ceramicsxe2x80x9d, wherein the abstract describing said session states that xe2x80x9cPoint defects occur in all ceramic materials, at larger defect concentrations to some extent bound in associates and clusters. Point defects are present within the bulk, at and near interfaces, however, not necessarily everywhere in equal concentrations. Many properties of ceramic materials and also processes involving such materials are strongly influenced by point defects. Therefore, detailed experimental and/or theoretical studies of point defects in ceramics and of related phenomena can significantly contribute to improve the current understanding of defect-related properties of ceramics and of many processes involving such materials.xe2x80x9d
This problem is especially acute when alumina pellets are used in ceramic armor, since non-uniformity of stopping power of adjacent pellets can have adverse effect on the reliability of the armor panel produced therefrom.
With this state of the art in mind, it has now been surprisingly discovered that by combining aluminum oxide with other oxides within specific parameter ratios, there is achieved an exceptional rise in the homogenity of the produced product in terms of parametric tolerance based on crush point studies of geometric bodies produced therefrom after sintering. Thus, it has been found that by using raw materials in which the chemical compositions fall within a specific range and forming them into geometric sintered shapes, homogenity of performance previously unknown in the art and quantitatively and qualitatively superior to that of products presently available on the market is achieved.
Thus, according to the present invention there is now provided a sintered, alumina ceramic product comprising about 90-97.5 w/w % Al2O3, about 0.5-1.0 w/w % MgO, about  less than 0.05-1.0 w/w % SiO2, about 4.5-7.5 w/w % ZrO2 and about 0.07-0.13 w/w % HfO2.
In preferred embodiments of the present invention there is provided a sintered, alumina ceramic product, comprising at least 0.585 w/w % MgO, 90 w/w % Al2O3,  less than 0.05 w/w % SiO2, 4.5 w/w % ZrO2 and 0.075 w/w % HfO2.
Preferably the sintered, alumina ceramic products according to the present invention, comprise up to 1.0 w/w % MgO, 97.5 w/w % Al2O3, 1 w/w % SiO2, 7.5 w/w % ZrO2 and 0.125 w/w % HfO2.
In an especially preferred embodiment of the present invention there is provided a sintered, alumina ceramic product, comprising about 0.6 w/w % MgO, 93 w/w % Al2O3,  less than 0.05 w/w % SiO2, 6 w/w % ZrO2 and 0.1 w/w % HfO2.
The ceramic products of the present invention can preferably include further minor amounts of additional oxides, selected from the group consisting of Na2O, P2O5, K2O, CaO, TiO2, Fe2O3, CuO, ZnO, BaO, Y2O3 and mixtures thereof.
Thus, in a most preferred embodiment of the present invention there is provided a sintered, alumina ceramic product comprising about 0.6 w/w % MgO, 92.62 w/w % Al2O3,  less than 0.05 w/w % Sio2, 6 w/w % ZrO2, 0.1 w/w % HfO2, 0.2 w/w % Na2O, 0.02 w/w % P2O5, 0.01 w/w % K2O, 0.1 w/w % CaO, 0.01 w/w % TiO2, 0.02 w/w % Fe2O3, 0.2 w/w % CuO, 0.02 w/w % ZnO, 0.5 w/w % BaO, and 0.04 w/w % Y2O3.
The present invention also provides a ceramic pellet for use in an armor panel, said pellet being made from a sintered, alumina product comprising about 90-97.5 w/w % Al2O3, about 0.5-1.0 w/w % MgO, about  less than 0.05-1.0 w/w % SiO2, about 4.5-7.5 w/w % ZrO2 and about 0.07-0.13 w/w % HfO2.
In especially preferred embodiments of the present invention there is provided an armor panel comprising a single internal layer of high density ceramic pellets which are directly bound and retained in plate form by a solidified material such that the pellets are arranged in a single layer of adjacent rows and columns wherein a majority of each of said pellets is in direct contact with at least six adjacent pellets, wherein each of said pellets is made from a sintered, alumina product comprising about 90-97.5 w/w % Al2O3, about 0.5-1.0 w/w % MgO, about  less than 0.05-1.0 w/w % SiO2, about 4.5-7.5 w/w % ZrO2 and about 0.07-0.13 w/w % HfO2 and there is less than a 30% difference between the crushing point of adjacent pellets.
The surprising homogenity and uniformity of the ceramic products according to the present invention enables the use thereof in electrically related applications.
Thus, the present invention also provides a ceramic product, as hereinbefore defined, wherein the bulk resistivity of a plurality of products prepared from the same batch exhibits a standard deviation of less than 0.1.
While the invention will now be described in connection with certain preferred embodiments in the following examples and with reference to the accompanying FIGURE so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of formulation procedures as well as of the principles and conceptual aspects of the invention.